1. Field of the Invention
The present invention relates generally to molding plastic bodies, and more particularly, to a method and apparatus for increasing the thickness of a molded plastic semiconductor package body without replacing or reworking the mold in which the body is molded.
2. Description of the Related Art
Many types of modern commercial semiconductor packaging techniques include encapsulating the components of the package in a dense, monolithic body of plastic, typically, a filled epoxy resin, to protect them from harmful environmental agents, e.g., moisture.
In the development, production, and evaluation of new, plastic-bodied semiconductor devices, it is frequently necessary to modify the thickness of the plastic body, or xe2x80x9cmold cap,xe2x80x9d of the device, e.g., to accommodate different thicknesses of die, substrate, lead frame, or combinations thereof. However, new molds typically cost upwards of $100,000 per set and require 12-18 weeks to fabricate, and may eventually be scrapped in the event it subsequently becomes necessary to change the thickness of the package body molded therein.
For developmental or short-run production purposes, a less-expensive modification in package body thickness can sometimes be achieved using an existing mold. Thus, a reduction in the thickness of a molded body can sometimes be achieved by making a judicious surface cut on one or both of the interfacial surfaces of the mold parts, so that they engage each other more closely and thereby reduce the height of the mold cavity. An increase in the thickness of the package body using an existing mold is more difficult and expensive, and involves welding new material on the interfacial surface of one or both of the mold parts to build them up, then machining away the new material to form new interfacial surfaces on the mold parts that engage each other at the desired new cavity height. In either case, however, the welding and/or machining of the mold parts can be expensive, and often warps the mold irreparably, thereby rendering an expensive mold unusable.
In view of this problem, a need exists for a simple, inexpensive, and reliable method and apparatus for temporarily modifying an existing mold to increase the thickness of body molded therein without the need to produce a new mold or rework an existing mold.
This invention provides a simple, inexpensive method and apparatus for temporarily modifying an existing mold to increase the thickness of a body molded therein, e.g., the plastic body of a semiconductor package, without the need for a new mold, or for reworking the parts of the existing mold.
The method comprises providing a mold having parts that engage each other at respective interfacial surfaces thereof to define a cavity within the mold, and a shim having a selected thickness and an opening through an interior portion thereof. The shim opening has a periphery corresponding to the periphery of the cavity of the mold. The shim is interposed between the interfacial surfaces of the mold parts such that they are spaced apart by the thickness of the shim, thereby increasing the height of the mold cavity by the thickness of the shim. Thus, the length and width of the mold cavity thus remain the same, while the height of the cavity, and hence, the thickness of a body molded therein, is increased by the thickness of the shim.
The shim of the invention can be fabricated for a fraction of the cost of a new or re-worked mold chase, and can be obtained in a fraction of the time necessary to obtain either, and therefore represents an extremely cost-effective tool for the development, production, and evaluation of new semiconductor packages.